Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert energy from the solar spectrum more efficiently as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carriers are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
Intermixing at the surfaces of the cadmium sulfide and cadmium telluride layers can occur during the deposition process and the annealing process after deposition of the cadmium telluride layer. This intermixing can increase the minority carrier lifetime in the cadmium telluride, thereby increasing the open circuit voltage (Voc) and the fill factor, and therefore the efficiency of the device. However, intermixing caused by the annealing process is dependent upon several manufacturing variables including the annealing temperature, length of anneal, thin film and surface uniformity (especially at the p-n junction), dopant concentration(s), annealing atmosphere (e.g., humidity), etc. These manufacturing variables can create problems in forming substantially uniform PV devices during a large scale manufacturing process, resulting in varying efficiencies of the manufactured PV devices.
Thus, a need exists for cadmium telluride photovoltaic devices having improved p-n junctions defined by controlled intermixed layers of cadmium sulfide and cadmium telluride.